The present invention relates to devices for microwave-assisted chromatography. As generally recognized in the chemical arts, many chemical reactions can be initiated or accelerated by increasing the temperature—i.e. heating—the reactants. Accordingly, carrying out chemical reactions at elevated (i.e., above ambient) temperatures is a normal part of many chemical processes.
The benefit of using controlled microwave energy for elevating the temperature of a chemical reaction is well known. For example, U.S. Pat. No. 6,753,517 to Jennings, incorporated entirely herein by reference, discloses a microwave-assisted chemical synthesis instrument using tightly controlled microwave energy.
More recently, researchers have applied microwave assisted chemistry to the technique of chromatography. Chromatography in the present context includes liquid and gas chromatography. Of the two, the instrument of the present invention primarily relates to liquid chromatography, particularly as it pertains to solvent evaporation and sample preparation for elution.
Liquid chromatography is a technique utilized in both preparative and analytical chemistry. Liquid chromatography comprises a stationary phase interacting with a mobile phase. Typically, the stationary phase is a surface-active powder such as silica, alumina, or an inert size-separating material like a gel-permeation chromatography packing, or the like. This powder is contained in a chromatographic column. In preparative chemistry, the mobile phase generally consists of a reaction solvent and a chemical to be identified, analyzed, or purified. This is collectively referred to as a sample. The mobile phase carrying the sample is caused to migrate through the stationary phase. Depending on how the sample interacts with the surface characteristics of the stationary phase, different compounds will migrate through the stationary phase at different rates. Preparative chemistry is useful for identifying and purifying various chemical components in the sample using analytical chemistry. Analytical chemistry utilizes carrier solvents to move the sample through the stationary phase.
In some instances, the reaction solvent is not ideal for the further purification of the compound dissolved therein. Residual reaction solvent in the stationary phase can result in the problem of poor separation and recovery of the desired components during elution.
One solution to this problem is to utilize a rotary evaporator. This procedure, however, is time consuming and not entirely effective. For example, the use of larger chromatography columns having more stationary phase will require one or more hours to evaporate the reaction solvent.
Another solution to this problem is to apply heat with or without vacuum. This procedure, however, may result in the degradation of the stationary phase or melting of the chromatography column material.
Another solution to the problem of residual reaction solvent in the stationary phase is to flash away the solvent using microwave energy. Microwave energy is used to vaporize liquid in U.S. Pat. No. 4,330,946 to Courneya. The Courneya '946 patent discloses microwave energy to vaporize liquid from agricultural material, such as grain. The Courneya '946 patent further discloses the use of air inlets for introducing air to sweep across the material, aerosolize the vapor and carry it away via a vacuum pump. The Courneya '946 patent also states the need for an auger-driven process to insure proper agitation of the material, as well as to move the material through the apparatus. A heat reclaim mechanism assists the microwave energy drying process.
As previously stated, microwave energy has been applied to the technique of chromatography. For example, U.S. Pat. No. 6,029,498 to Walters et al. discloses the incorporation of microwave absorbing material into the chromatography column itself or positions adjacent the column. The chromatography sample contained therein is heated by the microwave absorbing material via conduction or convection. The Walters '498 patent further discloses, however, the use of a heated chromatography column for the purpose of enhancing the speed of separation and the consistency of elution times.
Another example of the application of microwave energy to liquid chromatography is demonstrated in U.S. Pat. No. 6,630,354 to Stone. The Stone '354 patent discloses a method of using microwave-induced dielectric polarization to enhance the diffusivity of a liquid or a supercritical fluid mobile phase in chromatography, while having essentially no effect on other physical properties of the mobile phase. The primary focus of the Stone '354 patent, however, is to increase the diffusivity of the reaction solvent and to combine the advantages of liquid and gas chromatography. Heating the reaction solvent to increase its diffusivity could be hazardous if the solvent is combustible. To overcome this, the Stone '354 patent discloses the use of microwave pulses. The Stone '354 patent further states that a microwave apparatus capable of delivering very short pulses of radiation is not available, and instead teaches the use of a conventional, non-cycling microwave oven.
Yet another example of the application of microwave energy to liquid chromatography is demonstrated in U.S. Pat. No. 6,649,051 to Jamalabadi et al. The Jamalabadi '051 patent discloses a method of processing a sample into a flow-through device containing a porous solid media and thereafter subjecting the device to microwave energy. As in the Stone '354 patent, the Jamalabadi '051 patent discloses the use of a conventional microwave oven and further discloses the use of a more precise microwave power source, such as one manufactured by CEM Corporation of Matthews, N.C., USA, the assignee of the present invention.
Yet another example of the application of microwave energy to liquid chromatography is demonstrated in U.S. Pat. Application Publication No. 20030205456 to Jamalabadi et al. The Jamalabadi application discloses a method of processing a sample comprising introducing a sample in a flow-through device containing a porous solid media therein. The flow-through device is defined as having walls and having an inlet end and an outlet end; a solid porous media disposed within the flow-through device including attached active components. After introducing a sample into the flow-through device, the device is subjected to a radiated energy source, such as microwave energy, prior to further chromatography steps.
Furthermore, none of the above-referenced patents or applications teaches or suggests the use of a dry-loading technique. A typical dry-loading technique includes addition of a given amount of chromatography media, for example silica or alumina, to a vessel holding the sample. The sample is preferably dissolved in a reaction solvent. Following absorption of the sample by the chromatography media, the reaction solvent is evaporated and evacuated, leaving dried chromatography media containing the sample. The dried chromatography media is then dry-loaded, either manually or automatically, onto a separation chromatography column for separation and identification of the sample components.
The advantage of a dry-loading technique lies in the flexibility it allows for evaporation of the reaction solvent. Accordingly, dry-loading streamlines the subsequent separation and analysis of the sample components. The dry-loading technique is infinitely adjustable for a given sample volume, as opposed to the limitations imposed by preloading a specific amount of chromatography media into a flow-through device, or a sample module, or the like. For example, if the flow-through device contains too little chromatography media for a given amount of reaction solvent, the chromatography media will not sufficiently absorb the solvent. Subsequent heating of the chromatography media to evaporate the solvent will result in a highly viscous mixture from which the sample cannot be salvaged. If the flow-through device contains too much chromatography media for a given amount of reaction solvent, subsequent separation and identification chromatography efforts will be confounded by the excess media, resulting in wide, overlapping peaks having poor resolution.